In electrical apparatuses having solenoid valves, motors, power supply circuits, or the like, dust cores produced by pressure-forming soft magnetic materials are used. A soft magnetic material is composed of a plurality of composite magnetic particles, and each composite magnetic particle is constituted by a metal magnetic particle and a vitreous insulating organic coating film covering the surface of the metal magnetic particle. The magnetic properties required for the soft magnetic material are to achieve a high magnetic flux density by application of a small magnetic field, and high sensitivity to changes in external magnetic fields.
In the case where a soft magnetic material is used under an AC magnetic field, an energy loss called “core loss” occurs. The core loss is a sum of a hysteresis loss and an eddy-current loss. A hysteresis loss is an energy loss caused by the energy needed to change the magnetic flux density of the soft magnetic material. The hysteresis loss is proportional to the operating frequency and is thus dominant mainly in a low frequency range. An eddy-current loss is an energy loss mainly caused by the eddy current flowing between the metal magnetic particles. The eddy current loss is proportional to the square of the operating frequency and is thus dominant mainly in a high frequency region. In recent years, electrical devices are required to achieve size reduction, higher efficiency, and higher output. In order to fulfill such requirements, the electrical devices must be used in a high frequency region. Due to this, it is particularly desirable to decrease the eddy current loss of the dust core.
In order to decrease the hysteresis loss in the core loss of the soft magnetic material, the distortions and dislocations inside the metal magnetic particles should be removed to promote movements of domain walls and to thereby decrease the coercive force Hc of the soft magnetic material. In contrast, in order to decrease the eddy-current loss in the core loss of the soft magnetic material, each of the metal magnetic particles should be completely covered with an insulating organic coating film to ensure insulation between the metal magnetic particles and to thereby increase the electrical resistivity ρ of the soft magnetic material.
A technique related to the soft magnetic material is disclosed in Japanese Unexamined Patent Application Publication No. 2003-272911 (Patent Document 1). Patent Document 1 discloses an iron-based powder (soft magnetic material) in which aluminum phosphate-based insulating organic coatings having high heat resistance are formed on surfaces of particles mainly composed of iron. According to Patent Document 1, a dust core is produced by the following process. First, an aqueous solution of an insulating coating film containing an aluminum-containing phosphate and a dichromate containing, e.g., potassium is sprayed onto iron particles. Then the iron particles sprayed with the aqueous solution of the insulating coating film were retained at 300° C. for 30 minutes and at 100° C. for 60 minutes. As a result, the insulating organic coatings on the iron particles dry, and an iron, based powder is obtained. Subsequently, the iron-based powder is pressure-formed and then heat-treated to produce a dust core.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2003-272911